After i.v. injection of 125I-labeled rat atrial natriuretic factor ([125I] ANF; 99-126) in tracer dose to mice, a saturable binding to lung membranes was evidenced using a filtration assay. Analysis of the membrane-bound radioactivity by high-pressure liquid chromatography indicated that it corresponded to the intact hormone in sinorphan-treated mice. [125I]rANF binding was inhibited completely by i.v. administration of rANF with an ED50 of 1.0 +/- 0.1 nmol/kg, a value obtained in sinorphan-treated mice. SC 416,542, an ANF analog with a four amino acid deletion in its ring, representing a selective ligand of ANF clearance receptors, was as potent as rANF in inhibiting the in vivo binding. By contrast, ANF fragments produced by enkephalinase (EC 3.4-24.11, membrane metalloendopeptidase) were less potent or even inactive in competing with [125I]rANF. It is concluded that [125I]rANF binding to lung membranes in vivo occurs to clearance receptors. [125I]rANF binding was enhanced by more than 2-fold in mice receiving enkephalinase inhibitors such as sinorphan and, although to a lesser extent, aminopeptidase inhibitors; on the other hand inhibitors of a variety of other peptidases were ineffective. These data confirm by a novel approach that enkephalinase plays a key role in the inactivation of circulating ANF. Hence, the in vivo binding test can be used to assess the activity of clearance receptor ligands and peptidase inhibitors, two classes of drugs affecting ANF metabolism, with potential clinical utility in cardiovascular and salt-retaining diseases.